1. Field of the Invention
The present invention relates to a detection sensor and a resonator preferably used to detect presence or absence of a substance having a mass and the mass of the substance.
2. Description of the Related Art
In accordance with the progress of the microfabrication technique such as the micromachine/MEMS (Micro Electro Mechanical Systems) technique, a mechanical resonator with a very small size has been obtainable. Thereby, the mass of the resonator itself can be decreased, and a high-sensitivity resonator is being realized in which frequency or impedance characteristics are fluctuated by a mass change due to attachment of a very small substance (such as molecule or virus) at a molecule level. Use of this high-sensitivity resonator makes it possible to constitute a sensor capable of detecting presence and quantity of a very small substance. That is, because the size of the resonator is greatly decreased, the frequency of the resonator is increased up to GHz level and moreover, Si can be used as the material of the resonator. Therefore, the study aiming at integration with a semiconductor circuit is being progressed.
Moreover, by using the way of generating vibration (vibration mode) having a small vibration energy loss due to air viscosity or the like, it is possible to cause the resonator to generate the vibration having a very high Q value (Quality Factor) even in air and accurate observation of the frequency fluctuation has been achievable.
As an apparatus for detecting the quantity of a substance based on the frequency change of a resonator, a QCM (Quartz Crystal Microbalance) sensor is known. This uses the property of a vibration frequency fluctuating (or lowering) depending on the mass of a substance when the substance attaches to a quartz resonator. The QCM sensor has a superior performance as a mass sensor for measuring a very small mass change. For example, the QCM sensor is frequently used as a thickness meter (vapor deposition monitor) (for example, refer to Non-patent Document 1: WHAT IS A QUARTZ CRYSTAL MICROBALANCE—QCM, Internet<URL: http://www.pharmaceutical-int.com/categories/qcm-technolo gy/what-is-a-quartz-crystal-microbalance-qcm.asp>).
Moreover, a technique has also been proposed which measures a mass change of a lipid bilayer formed on the surface of a quartz resonator by allowing molecules for providing a gustatory sense or olfactory sense to be adsorbed by the bilayer (for example, refer to Non-patent Document 2: Ikuo Okabatake, “To measure gustatory sense or olfactory sense by weight”, Analysis, The Japan Society for Analytical Chemistry, 2003, No. 10, pp. 606-609).
By using this system, it has been already reported that a hydrogen-gas detection sensor to which platinum or palladium is applied as a hydrogen-molecule adsorption film or alcohol component detection using a PMMA polymer or food smell detection can be realized.
However, in the case of the sensor using a resonator whose vibration characteristic is changed due to attachment of above-described very-small mass, there is always a demand for sensors of more sensitive, more compact, and lower in cost.
Moreover, in the case of the technique described in Non-patent Document 2, a lipid bilayer having a high adsorption property of molecules has been used. However, this bilayer indispensably requires moisture and the use thereof in a dry atmosphere is subjected to restrictions. Moreover, there are problems that it is difficult to freely micro-fabricate quartz and integrate the quartz with silicon. Because of these reasons, further ingenuity is necessary to improve the sensitivity.
In the case of a QCM sensor, a vibration frequency is shown by the following expression.fo=α/t 
In the expression, α denotes a constant for determining a frequency and t denotes the thickness of a quartz resonator.
Thus, because the vibration frequency of the QCM sensor is inversely proportional with the quartz resonator thickness, it is impossible to sufficiently decrease the thickness. This means that increase of the ratio between a very small mass to be detected and the effective mass of a resonator is limited in the QCM sensor.
Moreover, when constituting a sensor for detecting the mass of a substance and when other substance other than a substance of interest attaches to a resonator, detection accuracy is deteriorated. Therefore, there is a demand for sensor capable of performing high-accuracy detection independently of an environment in which the sensor is used.
The present invention is made in light of the above technical problems and its object is to provide a detection sensor with high sensitivity, small size, low price, and high accuracy.